Summary report on existing techniques, procedures and criteria for assessment of effectiveness of interventions

D2.3 FINAL version 31-07-2014

Lead beneficiary : TU Delft

Participants: all

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SUMMARY REPORT ON EXISTING TECHNIQUES, PROCEDURES AND CRITERIA FOR

ASSESSMENT OF EFFECTIVENESS OF INTERVENTIONS

1. Introduction ..................................................................................................................................... 3

2. Methods and techniques based on the measurement of the moisture content in the wall .............. 4

2.1 Low invasive techniques ................................................................................................................ 5

2.1.1 Gravimetric method ............................................................................................................... 5

2.1.2 Sandrolini & Franzoni method .............................................................................................. 10

2.1.3 Calcium-carbide method ...................................................................................................... 11

2.2 Non-invasive techniques ............................................................................................................. 12

2.2.1 Capacitance moisture meters .............................................................................................. 12

2.2.2 Resistance moisture meters ................................................................................................. 13

2.2.3 Neutron probe ...................................................................................................................... 14

2.2.4 Georadar ............................................................................................................................... 14

2.2.5 Microwave ............................................................................................................................ 15

2.2.6 NMR ...................................................................................................................................... 16

2.2.7 Infrared (IR) thermography .................................................................................................. 17

2.2.8 Ultrasound ............................................................................................................................ 18

2.3 Conclusions on methods based on the assessment of the moisture content ............................ 18

3. Other methods for the assessment of effectiveness of interventions .............................................. 20

4. Criteria for the assessment of the effectiveness of interventions .................................................... 21

5. Conclusions ........................................................................................................................................ 23

Literature ............................................................................................................................................... 24

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1. Introduction This report aims to provide a short critical overview of the existing techniques, procedures and criteria for the assessment of the effectiveness of interventions against rising damp. Most of the existing methodologies for the evaluation of the effectiveness of an intervention against rising damp are based on the comparison of the moisture content in the wall before and after the intervention: the moisture content should significantly decrease for an intervention to be judged as effective. In section 2 different common techniques for the investigation of the moisture content in walls are discussed, including their advantaged and limitations. Because of the long time (from several months to few years ) necessary for a treated wall to dry, investigation methodologies alternative to the measurement of the moisture content have in the past been developed for the assessment of the effectiveness of an intervention within a short term from the application. These methodologies are discussed in the section 3. Criteria for the assessment of an intervention against rising damp are discussed in detail in section 4.

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2. Methods and techniques based on the measurement of the moisture

content in the wall The most common way of assessing the effectiveness of an intervention consist in measuring the moisture content in the wall before and after the intervention. A significant decrease of the moisture content identifies an effective intervention. Because of the long time (from several months to few years ) necessary for a treated wall to dry, the moisture content after intervention should be assessed after a significant period of time from its execution (e.g. 1 or 2 years). Preferably, the moisture content before and after intervention should be assessed in the same period of the year, in order to minimize the effect of seasonal variations in the moisture content in the wall. In the following sections an overview of the most common techniques for assessing moisture content is given, including their advantages and limitations. The techniques have been subdivided in low-invasive and non-invasive. Non-invasive techniques should be preferred whenever possible, eventually supported by low-invasive techniques. Invasive techniques should be avoided in the case of monumental buildings, unless strictly necessary for the understanding of the damage phenomena.

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2.1 Low invasive techniques

2.1.1 Gravimetric method

The so called “gravimetric method” is generally considered as the most reliable method for the determination of the moisture content. Gravimetric means ‘relating to the measurement of weight’[5]. The method consists in sampling of powder or solid pieces from the masonry followed by gravimetric determination of the moisture content of the sample after drying. Because of the size of the samples needed, this method can be considered as low-invasive. The European Committee for Standardisation (CEN), Technical Committee for Cultural Heritage (TC346) in the recently established standard focused on the measurement of MC in cultural heritage materials [6], recommends weighing (EN 322:1993 and EN 13183-1:2002) next to conductivity (EN 13183-2:2002) and capacitance (EN 13181-3:2005) as one of the methods for the assessment of the moisture content. In the following paragraph a procedure for the gravimetric assessment of the moisture content and distribution in walls is described and commented. A detailed description of the sampling and interpretation of the data is given as well. Sampling The sampling involves extraction of material from the wall by means of dry drilling. A solid drill or a hollow drill (core drilling) can be used. The Italian Recommendation NORMAL 40/93 reports: “the use of low speed drilling for material sampling is recommended in order to avoid samples heating and moisture loss by evaporation, as well as shrewd sampling points choice and proper standardisation of the whole measuring procedure” [7]. Following to this recommendation, research has shown that the rotation speed of the drill may have an effect on the measured moisture content in the case of relatively hard materials with low porosity, as e.g. Serena sandstone, while no significant effect is observed for relatively weak material with higher porosity, as fired clay bricks [8].

Drilling should be carried out at different heights (e.g. 0.2, 0.5, 1, 1.5 m up to the undamaged area) and depths (0-20 mm, 20-50 mm, 50-100 cm up to the middle of the wall) along a vertical profile (figure 1) [9]. As the moisture content depends also on the type of material, it is recommended to sample in the same type of material (brick, stone or mortar) at the different heights. Information on the type of material and on the presence of damage should be reported too, in order to correctly interpret the results. The samples, which should have a weight of at least 1-2 g, are collected in bottles or plastic bags, which are hermetically closed and transported to the laboratory. When the sampling is repeated after an intervention against rising damp, with the aim of assessing its effectiveness, it is important that this takes place on the same brick/stone unit drilled before the intervention, since the heterogeneity of (historical) building materials may lead to relevant differences in their moisture content.

Figure 1 Sampling at different heights in the wall

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Drying The samples are dried in laboratory in an oven at a temperature of

40 C (in case gypsum is present) , 60 C (in the case of organic

material) or 105C until the constant weight is reached. The drying can also be carried out on site by a portable thermo-balance (figure 2). These balances, however, can lead to errors due to poor experience of the operator (parameters setting, sample handling, etc.) or high sensitiveness of the balance (wind, slope and vibrations can often affect the results) [10]. Moreover, the time needed to dry each sample separately makes the procedure time consuming. Drying in laboratory is therefore preferable. Calculation of the actual moisture content (MC) The term “actual” moisture content is used in this document to distinguish the moisture content actually present in the wall at the moment of the sampling (actual moisture content, MC) from the moisture content due exclusively to the hygroscopicity of the material (for very fine porous materials) and to the presence of hygroscopic salts (hygroscopic moisture content, HMC) The actual moisture content is derived from the difference between the weight of the sample before and after drying, and it is expressed in w%.

Measurement of the contribution of hygroscopic salts to moisture content If hygroscopic salts are present in the wall, these can adsorb moisture from the air, when the RH of the air is higher than the RH of equilibrium of the salt (equilibrium RH of common salts at ambient temperature can be found in [11]). When this condition is verified, the wall will have a moisture content (MC) which is partially determined by the amount and type of hygroscopic salt and the RH of the air.[12] Therefore, for a complete and sound assessment of the moisture content, and thus effectiveness of an intervention, both the actual moisture content and the hygroscopic moisture content (HMC) should be measured. The HMC can be measured by storing the dry samples (the same samples as used for the MC measurements) in a climatic box at a defined RH (figure 3), until constant weight is reached. In practice, it is not always possible to wait until the constant weight is reached, especially when a large amount of very hygroscopic salts are present, since this may take several months; in these cases a period of 4 weeks is, in the authors’ experience, enough to get a reliable indication. Generally, hygroscopic moisture uptake at a very high RH (96 ± 1 % RH) is preferred, in order to include the hygroscopic adsorption of all soluble salt types. Measurements at different RH’s might be performed to spot the presence of specific salt types.

Figure 2 Use of portable thermo-balance on site (L. Miedema, 2014)

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The HMC96% is derived from the difference in weight between the dry sample and the sample after

conditioning at 20C 96 ± 1 % RH:

A high HMC, higher than a value which depends from the hygroscopicity of the material itself (see table 1 for reference values), indicates the presence of soluble salts. The HMC gives therefore an indication of the amount of hygroscopic salts present in the material: the higher the salt content and/or the more hygroscopic the salt type, the higher the HMC. The use of the HMC for indicatively assessing the salt content has some advantages with respect to conductivity measurements, another relatively simple method for the assessment of the presence of salts:

• Cheaper and of easier execution than conductivity measurements; • Less labour intensive, in case several samples need to be measured; • The samples are not altered and can be used for further investigation; • It is reliable also for CaCO3 and Ca(OH)2 containing materials, whereas, in case of the use of

conductivity measurements, some dissolution of these components may affect the results [4].

• Results can be easily coupled to MC measurements for the identification of the moisture source.

Material RH = 75% RH = 96%

Fired-clay brick 0.25 0.5 – 0.7

Calcium silicate brick

0.5 - 2 3 - 6

Gypsum < 0.1 3.5

Cement plaster 0.5 2

Figure 3 Storing the dry samples in a climatic box in laboratory(L. Miedema, 2014)

Table 1 Hygroscopic moisture content (weight %) of some materials at the RH of 75% and 96%

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Ion Chromatography and X-Ray Diffraction (XRD) Whenever a precise determination of the soluble ion (salt) content is needed, Ion Chromatography can be used (figure 4.1). Ion chromatography is a process that allows the separation of ions and polar molecules based on their affinity to the ion exchanger. The technique is used to measure the concentration of the soluble ions present in a sample. The amount of every measured ion is generally expressed as a weight percentage of the sample or in ppm. X-ray diffraction is another technique can be used to identify the types of salt (figure 4.2). X-Ray

Diffraction (XRD) is a method for the investigation of crystalline mixtures, giving information related

to the phase structure. In fact, each individual phase produces a characteristic spectrum, which

overlaps with spectra of others components. The interpretation of these overlapping spectra leads

to the identification of all phases present in the sample under investigation. The intensity of the

peaks is related to the concentration of the corresponding phase; this allows to perform a semi-

quantitative analysis on the crystalline phases present. The relatively high detection limit (3-4%)

makes this method suitable for analyses of salt efflorescence.

Interpretation of MC & HMC results

From the pattern of moisture and HMC distribution, the presence of rising damp and the importance of this phenomenon in the wall can be assessed. When the MC is higher than HMC96%, a moisture source (other than salt hygroscopicity) is present. In figure 5 some typical patterns of MC and HMC (dotted line) are shown. Figure 5.1 reports the MC & HMC distribution generally measured in the presence of rising damp: the MC is higher than the HMC in the lower part of the wall (at this height a moisture source other than salt hygroscopicity and air RH is present); the MC decreases from the lower to the upper part of the masonry. In the upper part of the wall, the HMC is higher than the HMC96%:. At this height the MC is determined by the combined presence of salt (accumulating here because of evaporation) and air RH. As a consequence the MC and HMC lines cross each other. When rising damp is present, normally the actual moisture content decreases from the inner to the outer part of the wall. On the contrary, the HMC increases from the depth to the surface, where salts accumulate because of evaporation. Similarly, because of drying, most salts accumulate at the upper fringe of the rising damp zone where the MC decreases, with the most soluble salts reaching higher levels in the wall (salt fractionation) [11].

Figure 4.1 Dionex Chromatograph (ISAC CNR, 2014) Figure 4.2 Philips PW 1730 diffractometer (ISAC CNR, 2014)

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1. Rising damp and salts 2. Rising damp and no salts 3. Salts and no rising damp

Generally, in a wall in which rising damp and salts are present, the pattern shown in figure 6 can be visually observed. Four different zones can be distinguished. In the lowest, wet zone, the wall is that wet that all salts are in solution and no damage occurs. Above this wet zone an area can be seen where salts crystallize at the surface as efflorescences (i.e. the moisture content is enough to keep the drying front at the surface). Higher in the wall, the drying front is present in the inner part of wall and drying takes place by vapour diffusion. The salts cannot be transported to the surface and will crystallize in the material (crypto-florescences), causing damage. Above the maximum height of the rising damp a dry zone is present without any damage [11] [13].

0 5 10 15 20

MC and HMC (%m/m)

0

100

200

300

400

he

ight

(cm

)

Depth

15-25cm

0 5 10

MC and HMC (%m/m)

0

100

200

300

400

500

he

igh

t (c

m)

Depth

15-25cm

0 5 10

MC and HMC (%m/m)

-50

0

50

100

150

200

250

300

350

400

he

igh

t (c

m)

Depth

15-25cm

Wet zone

Efflorescence zone

Crypto-florescence zone

Dry zone

Figure 5 Typical patterns of MC and HMC

Figure 6 Different zones rising damp [4]

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2.1.2 Sandrolini & Franzoni method

A method for measuring repeated times the moisture content in the wall is proposed by Sandrolini and Franzoni [10]. This method can be considered low-invasive and it seems particularly suitable in those cases where repeated sampling is not allowed. The method consists in creating a closed environment in a cavity in a wall where fragments of materials from the wall are inserted (figure 7). By recording the weight of the sample once equilibrium is reached (this will take at least one month), the effectiveness of the intervention can be assessed.

Figure 7 Schematic set up of the method for the measurement of the moisture content [3]

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2.1.3 Calcium-carbide method

Another method for measuring the moisture content in the wall is the calcium carbide method (figure 7). This method can be considered low-invasive. The calcium carbide method consists in drilling powder samples (usually a 10 gram sample is enough) and mixing this with a known quantity of calcium-carbide in a carefully closed vessel. Because of the following reaction: CaC2 +2H2O Ca (OH)2 + C2H2 acetylene (C2H2) is formed, which produces an increase of pressure in the bottle. By measuring this pressure increase by the use of a calibration curve, the moisture content can be derived [8].

This technique has the advantage that is relatively easy and it can be used directly in-situ. However, it has also some limitations:

- The method is not very accurate. Besides the errors related to the collecting of the sample, errors can be related to:

o insufficient pulverization and mixing. Only the water reacting with the calcium-carbide is measured.

o Leakage of the bottle. A small leakage will already have important influence on the pressure.

o Time for the reaction. In general the complete amount of water has reacted only after 10-30 minutes whereas it is not practical to wait longer than 1 minute.

o Temperature. The pressure developed by the reaction is influenced by the temperature.

- The powder sample cannot be used for other measurements (like assessment of soluble salts) [13].

Figure 8 A carbide meter kit containing a balance, carbide powder and a pressure gauge for measuring moisture in powder samples on site [2]

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2.2 Non-invasive techniques Non-invasive methods for the measurement of the moisture content in walls include capacitance moisture method, resistance moisture method, neutron probe, georadar, microwave method, Nuclear Magnetic Resonance (NMR), Infrared (IR) thermography and ultrasound The main advantage of these methods is the fact they in principle do not require the collection of samples of material.

2.2.1 Capacitance moisture meters

A popular method for measuring the moisture content in walls involves the electrical capacitance or dielectric constant of the building material. Capacitance moisture meters work on the principle of variation of the dielectric constant of a material in presence of water. The dielectric constant of a material is its ability to hold an electric field. Water has a very high dielectric constant: approximately 80.1 at 20°C. By contrast, dry building materials have dielectric constants of less than 5, so capacitance is proportional to the volume of water in the sample. Numerous hand-held meters are commercially available that use this property to estimate the moisture content in a non-destructive manner [14]. Hand-held meters are attenuation devices, composed of a box transmitter with contact and receiver electrodes. The transmitter sends a radiofrequency signal from the surface into the material, and the attenuation in this signal when it arrives at the receiver indicates the moisture content of the material. The meter is usually battery-powered so the signal is weak and does not travel far into building materials. The meter typically consists of two pads that are placed against the wall. The material in the wall forms a capacitor in a circuit between the pads that is highly sensitive to the moisture content of the material [14]. The main advantages of this method are its non-invasiveness and the easiness of use of the instrument. However, the method has some important limitations. First of all, a careful calibration is necessary for each material and measurement condition; the instrument cannot measure low moisture contents with precision. Moreover, the measured values are influenced by the presence of soluble salts; even if capacitance meters are less affected by the presence of salt than resistance meters, high salt contents have been shown to significantly affect the readings. [2] An important limiting factor in the use of this method is the contact between the instrument and the wall: it is important that the meter makes direct contact with the surface, because an air gap between the sensor and surface is likely to affect the reading. From this derives that the instrument might be of difficult use on rough and/or uneven surfaces, as often those of historic walls. Besides, it should be underlined that this method cannot be used to determine the moisture content at a particular location because the measurement location is not precise. All materials in the vicinity of the sensor will affect the reading, so it is difficult to pinpoint the exact source of a high or low measurement. From the above reported discussion it can be concluded that capacitance meters can be useful for a preliminary, non-invasive and qualitative survey of the moisture content in the wall (e.g. for detecting serious moisture problems such as leaks in walls and roofs), but they are much less suitable for the assessment of the effectiveness of an intervention against rising damp.

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2.2.2 Resistance moisture meters

One of the most popular techniques for determining the moisture content of building materials is based on measurements of the electrical resistance of the materials. Resistance meters work on the principle of decrease of the electric resistance of a material in presence of water. Hand-held resistance meters use two probes or pins, which are pushed in the material or wall surface. Some meters come with long pins requiring drilling of small holes into the wall. These instruments measure the resistance of a material to an electric direct current (dc). Since liquid water with impurities is a good conductor, the higher the moisture content of the material, the lower the electrical resistance. The presence of soluble salts significantly affects the measured values. Advantages of the methods are given by its non-invasiveness, the low cost of the instruments and the simplicity in using the method [14]. This method can be successfully used for the measurements of moisture content in wood, while it is much less suitable in masonry walls. This is because of multiple reasons:

- calibration is well established for most types of wood, since the measurements are reproducible if the material is consistent; differently, in the case of masonry the absence of homogeneity of the materials complicate the calibration.

- the pins can easily penetrate into wood, while they cannot straightforwardly be pressed into brick or stone elements.

Limits of the methods are given by the fact the measured values are strongly influenced by the presence of salt, the contact pressure applied to the pins, the temperature, the hardness and the irregularities of the materials. Moreover, the instrument measures only the outer layer of the material, which is strongly affected by the conditions at the surface as occurrence of surface condensation, evaporation etc.

From the above mentioned considerations, it is clear that resistance moisture meters do not provide a reliable account of the moisture content in masonry and cannot be considered suitable for the assessment of the effectiveness of interventions against rising damp.

Figure 9 Example of a resistance device (http://www.wagnermeters.com/flooring/wood-flooring/pin-moisture-meter/ )

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2.2.3 Neutron probe

Neutron scattering techniques work on the principle of the deceleration effect of hydrogen atoms on neutrons. The principle of the method is that if a specimen is irradiated with fast neutrons these will be slowed down by collision with the atoms of the specimen and a flux of slow neutrons will result. The basic assumptions of the method are that the slow neutron flux is dependent solely on the concentration of hydrogen atoms, instead of on other atoms, and that the relationship between slow neutron flux and concentration of hydrogen atoms is constant [15]. This method has the advantage to be non- destructive: repeated measurements can be carried out on the same location, allowing for monitoring of moisture content in time. The method is, up to a certain limit, not affected by the presence of soluble salts: only salts as gypsum or hydrates, providing additional hydrogen, can cause deviations. Disadvantages are given by: (1) the (low) radioactivity of the method, which raises considerable safety provisions [10]; (2) the need of specialized technicians for the interpretation of the results; (3) the variations of the analysis volume depending on the moisture content (when the masonry is drier, the analysis volume is greater, when the masonry is wetter, the analysis volume is smaller); (4) the need of careful calibration. Tests on a range of building materials have shown that the results can be seriously affected by changes in density and composition. Reliable results can be obtained only if the instrument has been calibrated on materials of the same density and composition as the structure under test [15]. This technique might be used for the assessment of the effectiveness of intervention against rising damp, next to low-invasive techniques.

2.2.4 Georadar

Radar is based on the principle that electromagnetic impulse waves make the water dipoles swing, which has a deceleration effect on the waves. Changes in rate, strength and frequency of the reflected radio signals reveal the presence of moisture. Ground penetrating radar can have a reach depth up to 1 m. [2, 14]. The main advantage of this method is its non-invasiveness. Disadvantage is given by the large deviations occurring in the presence of salt-rich masonry and metallic components. Besides, a careful calibration of the instrument is necessary. According to literature [2], this technique has not proved useful for measuring moisture in historic buildings because of the difficult interpretation of the measured data, requiring very accurate knowledge of variations in the materials beneath the surface. The method is therefore considered not very suitable for the assessment of the effectiveness of interventions against rising damp on existing buildings within this research project. The method might still be used on scale models of which the masonry structure and properties are known.

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2.2.5 Microwave

Methods based on microwaves make use of the change in dielectric properties of the material to determine the moisture content [14]. This technique is similar to some extent to the capacitance based methods: it measures the change in the dielectric constant of a material depending on changes in moisture content. However, differently from capacitance meters which use a radiofrequency signal, the microwave methods work through the propagation of electromagnetic radiation. The measurement is a combination of material density and moisture content. The microwave transmitter sends an electronic signal, and the amplitude and frequency of the response quantify the moisture content [2].

The method has the advantages of being non-destructive and not influenced by salt at a high frequency (>10Ghz). Disadvantages of the method are the following:

- The method does not enable the determination of the moisture content deep within the wall and only gives absolute moisture contents for materials which have been pre-calibrated by the manufacturer [16].

- The depth of penetration of the measurement differs in each material (a limited depth of penetration is measured in in very wet materials) and it is not possible to determine variations in moisture levels within this depth [2].

- Metal fixings or other metal elements within the wall will cause false readings [2]. From the above reported considerations, it can be concluded that the results obtained with this method can be considered as indicative and qualitative. This method can therefore applied for a qualitative assessment of the moisture content (e.g. in a preliminary survey) but it is not suitable to be used as only method in the assessment of the effectiveness of interventions against rising damp. The method might be used in combination with other low-invasive methods.

Figure 10 Example of a microwave moisture meter (http://www.trotec24.com/en-gb/measuring-instruments/humidity/t-600-moisture-meter-microwave.html)

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2.2.6 NMR

Nuclear magnetic resonance has the ability to detect a wide range of constituents, including water in different states. The technique works on the principle of the measurement of the magnetic resonance of the core of the hydrogen atom. A magnetic field is first applied to the specimen. When the field is released, hydrogen nuclei in the material gradually return to their ground-level energy state. The energy released by these atoms during decay is monitored, and the time for the decay is an indication of the amount of hydrogen. Since each water molecule consists of two hydrogen atoms, the amount of hydrogen in the sample is closely related to the amount of water. This technique provides accurate estimations of the moisture content of a material [14]. The application of NMR to the study of moisture and salt content and transport in building materials is a well-established technique in laboratory research [17, 18]. Differently, on-site application of NMR to Cultural Heritage is rather recent [13, 19, 20]. A breakthrough in the application of this technique has been the development of a portable NMR instrumentation, which allows one to investigate large objects in situ without any sampling. Main advantages of NMR are: (1) the quantitative values which can be obtained and (2) the fact that moisture measurements are not influenced by the presence of salts. Disadvantages of this technique are given by:

- the need of specialized personnel for the interpretation of the results; - the (still) very small depth of penetration; - the need of a very good contact of the NMR with the surface (this can prove difficult on

rough historical walls); - the difficulty of measuring in materials and/or structures where iron or other metal

contaminants are present [2] - the costs of the apparatus.

Based on the above mentioned considerations, the portable NMR can be considered a possible method for the measurement of the moisture content in walls and therefore for the assessment of the effectiveness of an intervention against rising damp. Actually, the availability of this instrument and the highly specialized personnel required for its use, will probably hinder the application of this technique in the present research project.

Figure 11 Use of portable NMR Mouse (http://www.nmr-mouse.de)

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2.2.7 Infrared (IR) thermography

Infrared thermography is based on the principle that water has a high thermal capacity, so change in moisture content can alter the thermal storage and transmission of a permeable material. IR thermography has been applied for more than 30 years to the monitoring in qualitative or quantitative way [21]. IR thermography can detect damp areas in the structures due to their different temperature compared with the dry ones. An infrared camera creates images indicating differences in temperature across a wall surface, which indirectly identify zones of moisture. This technique can therefore provide an indication of the presence of moisture and locate it, but cannot measure the moisture content. The technique has the advantage of being non-invasive, cheap, of easy use and relatively quick; all these factors make this technique very suitable for a first survey of the building and the identification of the presence and location of moisture problems. IR thermography has the following limitations:

- A quantitative measurement of moisture content in a wall or material is not achievable by this technique, due to the very different emissivity of dry construction materials and the need of detailed case by case laboratory calibrations.

- IR techniques can provide information only about the hygrometric state of a thin layer of material just beneath the surface; this layer is strongly affected by the outdoor conditions and it has different conditions from the inner part of the wall [3]

- IR techniques require a careful interpretation of the results, as different materials, shadows and features of the building can interfere with the measured surface temperature.

- It may be necessary to modify the surrounding conditions, for example by heating a wall or imaging on a wet day or in the dark in order to obtain informative results. [2]

- The choice of the right time when performing the test is very critical.

On the basis of the above reported consideration, it can be concluded that the method can be used to map qualitatively the moisture distribution due to the cooling effect of water evaporation, provided a careful analysis of the data is carried out [21]. The method can be used as support to quantitative methods in the monitoring of the effectiveness of intervention against rising damp.

Figure 12 Use of thermography (BBRI)

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2.2.8 Ultrasound

Ultrasound techniques use sound waves above an audible frequency to detect moisture in buildings, based on the fact that water transports sound waves better than air or solid materials. Generally, a sound generator is placed on one side of the wall and a listening device is placed on the other side of the wall. The listening device records the received signal and translates this to an audible sound corresponding to the signal strength. This method has the advantage of being non-invasive. The main disadvantage of the method is given by the fact that inhomogeneity’s as voids, cracks, presence of different materials in the wall cause major interferences [2]. This considerably hinders the use of this technique for the detection of moisture in historic masonries. Besides, another problem can be given by the difficulty of placing the sound generator and the listening device in corresponding places on the two sides of the wall, in case of difference in height of the ground level.

2.3 Conclusions on methods based on the assessment of the moisture

content In table 2 the properties of methods for the assessment of the moisture content in wall are summarized, in order to point out advantages and limits. Besides, the availability of the techniques of the research centres involved in this research project and the practical experience of the partners with these techniques are reported. This table aims at facilitating the selection of the most appropriated investigation techniques for the assessment of the effectiveness of the interventions in this research project.

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Gravimetric method

Sandrolini& Franzoni

Calcium-carbide

Capacitance meter

Resistance meter

Neutron probe

Georadar

Microwave

NMR MOUSE

IR thermography

Ultrasound

Type of method

Low-invasive Low-invasive

Low-invasive

Non-invasive

Non-invasive

Non-invasive

Non-invasive

Non-invasive

Non-invasive

Non-invasive

Non-invasive

Depth of penetration

Multiple depths

Multiple depths

Multiple depths

Surface Beneath the surface

In depth?

In depth

In depth

surface surface In depth

Area Spot, drilled or core

Spot, drilled

Spot, drilled

spot spot spot covers large area

spot spot Covers large areas

covers large areas

Measurement time

1 week off site

1 month after insertion

10 minutes

Immediate

immediate

immediate

Immediate

Immediate

immediate

immediate

Immediate

Components Balance, oven/climatic box, air tight bags/bottles, drill

Drill, rubber stopper, plasticine

Calcium-carbide kit

Meter unit unit unit Meter unit Infrared camera

Sensor, transmitter, unit, headphones, tubing

Cost Balance €€ Oven/climatic box €€€€

€€€ Meter €€€ CaC2 €€

€€€ €€€ €€€ €€€€ €€€€ €€€€€€ €€€€ €€€

Can it be used on site?

Yes (portable thermo-balances) / no (laboratory (preferable))

yes Yes Yes yes yes Yes yes yes Yes yes

How portable is the equipment?

Not portable Not portable

Portable Portable, battery operated

portable Portable (5kg) battery operated

Portable (0.2 kg), battery operated

portable

Portable (2kg), battery operated

Portable (0,5kg) operated

Do salts influence the measurement?

No, Measured hygroscopicity relates to salt content

No? No Yes, minimally

yes no yes no no no no

Can it detect or identify salts?

Yes, with additional tests on the samples

Yes, with additional tests on the samples

No no no no no no no no no

Easy of use & interpretation

easy easy Easy, interpretation based on knowledge of material

Easy to map areas, interpretation misleading

easy Specialist technicians

Difficult, require interpretation

Easy to map areas

Expert interpretation

Easy to map areas, can be misleading

Easy to map areas

Level of operator skill

basic basic basic basic basic expert

expert basic expert expert basic

Affected by material discontinuities?

no no no yes yes ? yes yes yes yes yes

Have partners access to this technique?

Yes (ISAC-CNR)

Yes? ? yes? Yes? ? ? yes No? Yes (ISAC-CNR)

?

Have partners experience with this technique?

Yes (ISAC-CNR)

Yes ? yes? Yes ? ? ? ? Yes (ISAC-CNR)

?

Table 2 Overview of methods (based on [2])

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3. Other methods for the assessment of effectiveness of interventions The time needed for a treated wall to dry varies from months to years, depending on the thickness of the wall, the initial degree of saturation and the drying conditions (temperature, Relative Humidity, air speed). This fact constitutes generally a limit to the assessment of interventions against rising damp. Because of the low invasiveness of the assessment (which would damage the newly restored wall) and because of the lack of long term guarantee of the intervention (which hinders any refund), an assessment of the effectiveness of the intervention is seldom carried out. The possibility of assessing the effectiveness of an intervention directly after its execution (instead of after few years), would promote the assessment of the quality of an intervention. Because of the above mentioned reasons, attempts have been made to develop a test which can assess the effectiveness of an intervention against rising damp, within a short time from its execution. For example, in the case of chemical injection, a method consists in sampling large cores (diameter about 10 cm) from the complete depth of the wall, drying them in laboratory and measuring their capillary water absorption [22] [23].

This method has some important limitations:

- The test can be considered destructive, because of the size of the drilled cores. - The drilled cores are fully dried before their water absorption in determined; the injection

products can therefore easily react. This is not the case in the practice, where the moisture supply after injection; in practice the product should therefore be able to react in a (partially) saturated wall. Recent research has shown that the presence of this drying phase in laboratory experiments can lead to a not fully reliable reproduction of the behaviour of the product in practice [24]. Nevertheless, the test still provides a very useful evaluation of the spreading of the products.

Recently, a modification of this method has been proposed in which no drying of the cores takes place but a controlled and constant supply of moisture is provided, similar to that present in the practice [24]. This method might be suitable for assessing the spreading and effectiveness of chemical products. Another method, suitable also in the case of interventions other than injections, consists in using tracers, as e.g. Li. After the intervention, the tracer is inserted at the base of the masonry. After a certain period of time, necessary for capillary transport of the tracer, the presence of the tracer in the wall is measured by collecting samples from the wall. The presence of the tracer indicates that capillary transport has taken place. The main limit of this method consists in the difficulty in identifying the presence of the tracer, as its amount might be very low after it has diluted in the water saturated wall.

Figure 13 Core drilling of core samples

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4. Criteria for the assessment of the effectiveness of interventions When evaluating the effectiveness of an intervention against rising damp, criteria should be defined. In the following paragraph an attempt in this direction is made. The defined criteria, however, should not be considered as definitive, but they might still be modified in the light of the results of this research project. An intervention against rising damp can be defined as effective if:

1. It leads to a significant decrease of the moisture content in the area above the intervention and/or to a lowering of the rising damp front. If this is a straightforward criterion, much more difficult is the definition of technical requirements, i.e. of values of reduction in moisture content for which the intervention might be considered effective. These values might probably vary depending on the intervention method: from a mechanical interruption it is expected that the wall will dry until a moisture content is reached, which corresponds to the hygroscopic equilibrium of the material with the environment. For less invasive interventions, as chemical injection or electro-osmosis a significant decrease (above 60-70%) with respect to the initial moisture content can already be considered satisfying.

2. It does not cause damage to the building on which it has been applied, during and/or after its execution. For example, mechanical interventions should not lead to cracks or displacements in the structure. In the case of successful mechanical or chemical intervention, due to the fact that moisture supply is stopped or reduced, the area above the interruption will dry out. This may result into an increase in the amount of efflorescences above the treated zone. This effect, which may have consequences also for the damage, should be tackled by using a salt resistant or a sacrificial plaster above the treated zone . Alternatively, once stopped rising damp, desalination by poulticing can be used as side measure. Similarly, in the case of electro-based methods or of methods enhancing the evaporation (see DL 2.1), an enhanced drying takes place leading, in the presence of salt, to an increased transport and accumulation of salt in the wall, fact which might have negative consequences for the damage. In this case, as the source of moisture is still present, removing the salt by desalination does not offer a permanent solution. Salt resistant plaster might still be an option, if the salt and moisture amount are not too high.

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3. It leads to an improvement of the interior climate and comfort of inhabitants. One of the

reasons for an intervention against rising damp is the improvement of the indoor climate (RH and temperature) and comfort of the inhabitants. The effect on the indoor climate can be controlled by monitoring the temperature and RH before and after the intervention. In the Netherlands, the following four climate classes are defined, depending on the function of the building (table 3) [1, 25]:

Climate class

Moisture production

Type of building Vapour pressure pi [Pa]

Average Temperature T [

◦C]

Average Relative Humidity (RH)

I Buildings with low to zero moisture production

Warehouses of dry goods, garages, sheds, churches, moderately used gym

1030 ≤pi <1080 18◦C 50 – 52%

II Buildings with limited moisture production and good ventilation

Offices, shops (without humidification in winter)

1080 ≤pi< 1320 20◦C 46 – 56%

III Intensively used buildings and rooms with a moderate vapour production

Homes, schools, retirement homes, nursing homes, recreation buildings and buildings with minor humidification

1320 ≤pi< 1430 22◦C 50 – 54%

IV Buildings with a high vapour production

Moist industrial spaces, laundries, swimming pools, bathhouses, dairies, press, textile mills, spaces with forced humidification

pi≥ 1430 24◦C > 48%

Table 3 Climate classes [1]

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5. Conclusions In this report criteria and common techniques for the assessment of interventions against rising damp are reported, including their advantages and limitations. This overview constitutes the basis for the selection of a common procedure for the assessment of the effectiveness of the intervention methods tested in the project, which will be defined in DL 2.4.

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